In telephone engineering, it is usual practice to provide protectors at central offices for each incoming line. These protectors, which may be termed modules, combine protection against excessive voltages resulting from lightning, for example, with protection against sneak currents. Sneak currents are not strong enough to do any damage if they flow briefly, but may generate enough heat to char conductor insulation and do other damage if allowed to persist. The sneak currents are produced by voltages of relatively low magnitude as compared to the excessive voltages mentioned hereinabove and usually result from accidental interference between telephone lines and adjacent power lines.
Protection of a telephone line against excessive voltage is usually provided by a so-called spark-gap protector which generally includes a pair of spaced carbon electrodes or a gaseous discharge device. One of the electrodes is usually connected to ground and the other to the incoming telephone line. Should a high voltage be impressed on the line, it will bridge the gap between the electrodes and cause current to flow to ground, thus bypassing sensitive equipment which is associated with the line.
The second type of protection is commonly provided by a device that is referred to as a heat coil. The heat coil includes high resistance wire which is wound on a metal sleeve inside of which a contact pin is held in a predetermined position by a fusible bonding material such as solder, for example. Should excessive currents occur on the line and persist, sufficient heat will be generated by the wire to melt the solder and release the pin. A spring is usually provided which urges the released pin into electrical contact with a source of ground potential to ground the line and protect sensitive line equipment.
Inasmuch as a ring conductor and a tip conductor are associated with each telephone station apparatus, each telephone line requires two protector assemblies. A telephone circuit protector module shown in J. B. Geyer et al U.S. Pat. No. 3,573,695 which issued on Apr. 6, 1971, includes two protector assemblies enclosed in a single insulative housing. Spark-gap and heat coil subassemblies therein are held in abutting aligned relation by a single spring which is part of the normal transmission circuit. The spring also serves to propel a pin of the heat coil subassembly into engagement with a grounding circuit, which includes one of two carbon blocks, during the passage of excessive currents through the heat coil. In Geyer et al, the axis of each heat coil pin is aligned axially with the axis of its associated carbon blocks. To complete a fault current path to ground, the pin in the heat coil subassembly must be brought into contact with a carbon block in the spark-gap protector subassembly. This causes excessive heating of the spark-gap subassembly, which becomes part of the fault path, because of the relatively high resistance of the carbon blocks. The extension of a contact pin through voltage protection portions of the protector has precluded the use of gaseous discharge devices in place of carbon blocks. Gaseous discharge devices, which are commonly referred to as gas tubes, are desirable because of their longer lives and because they afford better control of the breakdown voltage.
In a protector module shown in U.S. Pat. No. 4,215,381 which issued on July 29, 1980, to R. F. Heisinger, gaseous discharge devices may be used inasmuch as the voltage protection portion of the protector is taken out of the fault circuit. When sufficient heat is transferred to the heat coil subassembly such as by a current fault, a fusible alloy melts to allow a spring to cause a heat coil flange to move and touch a laterally projecting tab of a ground terminal assembly. If a prolonged voltage surge occurs, there is an arcing over in the voltage surge limiter assembly, heat energy is transferred to a pin of the heat coil which engages a portion of the voltage surge limiter assembly, the fusible alloy is melted, and the spring moves the heat coil flange plate as before. However, the Heisinger protector module continues the use of a spring as part of the normal transmission and fault current circuits. At times, the presence of the spring in the voice frequency circuit may result in noise on the line. Also, because the spring moves slidably, insulating sleeves are disposed about the spring to prevent shorting.
A protector module in which a spring is not in the transmission circuit is disclosed in U.S. Pat. No. 4,168,515. When an excessive current increase occurs, a fusible alloy is melted to allow a bobbin on a pin of a heat coil assembly to be moved by the spring. This allows a cup, which is supported indirectly by the bobbin, to be moved by the spring to engage a plate to which the heat coil, line and central office pins are staked. As a result, a fault current path is established from the line pin through the cup to a ground plate.
The aforementioned prior art protector assemblies each include a seemingly excessive number of elements. A protector assembly having substantially fewer elements and adapted to include either gas tubes or carbon blocks is disclosed in U.S. Pat. No. 4,458,288 which issued on July 3, 1984 in the names of J. L. Chapman, Jr. et al. Each of two protector assemblies supported in a common housing includes a current protection subassembly which comprises a dielectric base and a line pin and a central office pin connected together electrically. A shunting element is disposed concentrically about the line pin and is secured to one end of the line pin in an initial position by a fusible material. A spring between a cup of each voltage protection subassembly and the housing maintains the voltage protection subassembly in engagement with the shunting element. The spring is effective when current flow exceeds a predetermined level that is sufficient to melt the fusible material to cause the shunting element to be moved to a position where it engages a portion of a grounding subassembly to establish a fault current path to ground. For a prolonged voltage surge, heat energy is transferred from the voltage protection subassembly to the shunting element and melts the fusible material to allow the shunting element to be moved as in a current overload mode.
In the above-described protector, the heat coil includes a sleeve which is bonded to one end of a cylindrically shaped pin by solder. After soldering, but before the winding of turns of a wire about the sleeve, the assembly is heat-treated to recrystallize the solder which had been annealed. Then the assembly is stored and creep tests of samples taken. As can be surmised, the heat treating requires additional time and increases inventory. What is needed and what seemingly is not provided in the prior art is a protector in which bonding of elements which are to function in an overload mode is not required.